Design And Tracking Control Of SMA-Based Flexible Lower Limb Exoskeleton Robot

The increasing demand for exoskeleton robots in medical,logistics and military fields has made exoskeleton-related technologies a hot topic in the field of robotics research.Most of the existing exoskeleton robots adopt rigid drive,in which the driver directly drives the exoskeleton rod.This drive method has the advantages of simple control and high accuracy,but its rigid connection between driver and actuator leads to poor human-machine adaptability and weak environmental adaptability.The variable stiffness exoskeleton developed for the shortcomings of rigid actuation often uses dual motor control,which can change the joint stiffness and torque in real time and improve the man-machine adaptability of the exoskeleton,but also significantly increases the structural complexity and is not conducive to the miniaturization of the exoskeleton.Therefore,it is of great practical significance and application value to study a compact and flexible lower limb exoskeleton robot.This thesis first analyzes the physiological structure and movement characteristics of the human lower limbs.Based on the flexible driving principle,a variable stiffness series elastic actuator(SEA)using shape memory alloy(SMA)and the overall structure of the exoskeleton robot are designed.Then,the mathematical model of the SMA driver is analyzed,and a modified Brinson phase transformation model is proposed to solve the problem of the sum of crystal volume fractions not equal to one under certain constrained conditions in the Brinson constitutive model.A self-disturbance rejection controller(ADRC)is used to compensate for the system’s hysteresis.The exoskeleton robot is analyzed for kinematics and dynamics using theoretical calculations and virtual simulation methods to lay the foundation for controlling the exoskeleton.Finally,a fuzzy PID control method is used to control the exoskeleton and to study the effect of the displacement of the SMA driver output on the response of the exoskeleton system.The main research of the thesis is as follows:(1)Based on the principle of variable stiffness by changing the pre-compression amount of the spring,a variable stiffness and flexible SEA driver by SMA is designed.According to the physiological structure and movement characteristics of the human lower limbs,various joint structure schemes are determined to complete the overall structure design of the exoskeleton robot.(2)The issue of the sum of crystal fractions not being equal to one in the Brinson phase transformation model when SMA is in a partially constrained state is analyzed.Two phase transformation processes,namely heating-cooling and constant-temperature stretching,are used to describe the phase transformation endpoint of SMA.The austeniteto-martensite phase transformation equation in the original phase transformation model is modified,and a modified Brinson phase transformation model is proposed and compared with the corresponding experimental results.(3)A mathematical model of the SMA driver is established in MATLAB/Simulink.By analyzing the state-space equations,the order and input gain of the driver system are obtained.A self-tuning regulator controller(ADRC)is used to achieve precise control of the SMA driver system by using an extended state observer(ESO)to estimate and compensate for internal and external disturbances in real-time.(4)The kinematic and dynamic analysis of the exoskeleton system is conducted using the D-H method and Lagrange method to obtain the motion and mechanical characteristics of the exoskeleton robot.A dynamic model of the exoskeleton robot is established in ADAMS,and the kinematic and dynamic theoretical analysis results are validated through gait simulation of the exoskeleton to lay the foundation for the tracking control of the exoskeleton system.(5)Based on the fuzzy PID control method,the assistive characteristics of the exoskeleton robot are verified through joint simulation using MATLAB and ADAMS.Meanwhile,the influence of different SEA stiffness on the assistive characteristics of the exoskeleton robot is analyzed,and the system response is compared to validate the effectiveness of the designed lower limb exoskeleton robot in this study.